1. Field of the Invention
The present invention relates generally to concrete building foundations, and, more particularly, to a building foundation for use with expansive soils that includes a slab and wall assembly in which the walls are supported by the slab and/or an external sump pit for which access is provided in a window well or other access point external to the foundation walls and/or includes a dam about the external, lower portion of the slab and wall assembly for maintaining a void space under the walls and the slab.
2. Relevant Background
Commercial and residential buildings are often built on foundations comprising vertical perimeter walls of poured concrete. Since the vertical foundation walls are structural members that support the building, they are usually several feet in depth and function as beams bridging between footers or piers resting on bedrock or stable soil. It is common practice in such buildings to provide a basement, or ground floor, wherein at least a portion of the basement walls include the vertical foundation walls and wherein the basement floor is a poured concrete slab resting on the soil enclosed by the foundation walls. Typically, the foundation is constructed by first excavating a pit for the basement and for the foundation footers. Then, forms are erected around the periphery of the pit and concrete for the foundation walls is poured into the forms. The floors are next formed by pouring concrete onto a form supported by the soil and/or by the side walls. In other words, the slab or flooring is typically mechanically attached at one end to an inner side of the foundation walls.
A major problem with conventional construction in certain soil and climate conditions is that the slab floor is unstable due to movement of the underlying soil. Expansive soils are prevalent in many areas of the United States and other countries. These expansive soils can expand and contract considerably as a result of cyclical changes in moisture content or as a result of freezing and thawing cycles. The soil expansion and contraction problem can be especially severe when the floor is simply a slab of concrete poured onto the surface of the soil that forms the floor of the excavation pit. For example, certain dense clay soils tend to dry out after excavation and then later absorb water and swell. This swelling or expansion causes the slab to move relative to the foundation walls which can generate large forces that are sufficient to crack or break the slab. In general, because the foundation walls must support the building, they are supported by piers or pads on solid ground or bedrock or piers or pads on footings and therefore are very stable. However, the movement of the slab mechanically attached to the side of the foundation wall can readily damage the relatively rigid walls.
A variety of techniques have been implemented to control the effects of expansive soils on concrete foundations and structural slabs or floors. Generally, each of these techniques attempts to separate the foundation walls and structural slabs or flooring from the heaving soils or to at least absorb some of the expansive forces created by the moving soil. To address the problematic soils, such as Bentonite clay, builders have employed techniques such as raised, suspended, or free-spanning floors. Unfortunately, these techniques have proven to be costly, to increase the complexity of fabricating concrete foundations and flooring, to cause long-term structural or safety problems, and to reduce spacing between the floor and ceiling. Additionally, to obtain a particular wall height, a taller or bigger side wall is required to accommodate for the thickness of the floor slab and/or for void space provided under the floor slab. This requires addition material costs for concrete and labor costs for excavating and fabricating the foundation walls.
A common technique of protecting the foundation and slab from the expanding soil is to create a void space under the concrete slab or floor. To create the void, cardboard forms or other degradable material forms and/or removable forms are positioned under the form or pan used during pouring of the foundation walls and floor. With time, the material of the void form begins to deteriorate creating a void in which the soil can expand without moving the wall or floor. However, the degradation of the forms typically is accompanied by mold growth and the release of associated toxins, which can result in safety issues within the structure above the concrete foundation. Additionally, jobsite delays and inclement weather during initial construction can result in premature degradation of the cardboard void form and loss of the strength needed to support the curing concrete wall and floor.
Other difficulties that face the designer of a foundation are how to maintain the integrity of the void space underneath the walls and floor slab and how to maintain the strength of the foundation. When a void space is provided under a foundation wall and/or under a concrete slab attached to the side of the wall, excavated soil has a tendency to fall or migrate horizontally from excavated earthen walls and backfill into the void. This can lead to expansive soils being in contact, at least locally, with the foundation wall and/or floor, which can result in heaving or at least additional stresses in the foundation. The strength of the floor is often deteriorated by providing openings for utilities, such as plumbing and a sump pit used for draining collected groundwater from the area around the foundation. For example, sump pits are often placed as part of forming the slab for the floor or foundation slab, which causes a reduction in the slab strength in the vicinity of the sump pit that can result in cracking or failure of the slab. Additionally, the sump pit may itself be in contact with expansive soil that applies force against the pit and the pit walls in turn apply a force against the adjoining portion of the foundation slab. If provided simply as an opening in the slab, the sump pit may provide a path for molds and the like to enter the space above the slab, i.e., the commercial or residential building.
Hence, there remains a need for a foundation design that accounts for expansive soil but that also provides a relatively inexpensive method for manufacturing the foundation walls and flooring slab. In such a foundation design, preferably a void is provided under the flooring slab to control stresses caused by expanding and contracting soil and even with the void the strength of the foundation walls and slab are maintained. Further, it is preferable that the foundation design be such That standard (e.g., not taller) walls may be utilized to obtain a desired floor to ceiling height.